In a conventional method of synthesizing melody, a melody is first divided into a tone wave and an envelope wave and then the two waves are multiplied together to form a synthesized sound wave. This is shown in FIGS. 1-3. FIG. 1 shows a tone wave 10 plotted as a voltage vs time curve. An envelope wave is shown in FIG. 2 which is also plotted as a voltage vs time curve. Multiplying the tone wave 10 in FIG. 1 by the envelope wave 20 in FIG. 2, a composite sound wave 30 is obtained which is shown in FIG. 3. It should be noted that the composite sound wave 30 shown in FIG. 3 is obtained under ideal conditions, i.e. it is a hypothetical waveform.
In reality, the composite waveform 30 is more likely to resemble the shape of the waveform 40 shown in FIG. 4. This departure from the ideal form is mainly caused by the lack of adjustment for the DC voltage offset.
Traditionally, a melody can be synthesized by two methods. The first is a digital synthesizing method in which the digital data of the tone wave and the envelope wave are multiplied together by using a multiplier. The product of the multiplication is then sent to a digital/analog converter in order to complete the synthesis of the melody wave. The shortcomings of this method is that it requires the additional component of a multiplier and a higher operating frequency range for the conversion system.
The second method of synthesizing a melody wave is the analog synthesizing method. However, due to the lack of adjustment for the voltage offset and the lack of adequate positive/negative signal processing of the digital/analog converter, a distorted sound wave is frequently generated. This is shown in FIG. 4. The sound wave 40 shown in FIG. 4 contains a variable DC offset which is frequently the cause of an undesirable "pop" noise during the playback of the melody. In order to eliminate the "pop" noise, at least one coupling capacitor must be used to eliminate the DC component. This in turn increases the cost of the circuit.
It is therefore an object of the present invention to provide a digital/analog converter circuit of the analog multiplying type that does not have the shortcomings of the prior art digital/analog converter circuits.
It is another object of the present invention to provide a digital/analog converter circuit of the analog multiplying type that utilizes an analog synthesized circuit.
It is a further object of the present invention to provide a digital/analog converter circuit of the analog multiplying type that utilizes an analog synthesizing method and a DC offset compensator to compensate for DC offset.
It is yet another object of the present invention to provide a digital/analog converter circuit of the analog multiplying type utilizing an analog synthesizing method and a DC offset compensator to compensate for DC offset and to eliminate noise formation.